Steel composition, method for making same and parts produced from said compositions, particularly valves

ABSTRACT

This invention relates to a steel composition comprising, expressed in percentages by weight: 0.25-0.35% C, 24-28% Cr, 10-15% Ni, 3-6% Mn, 1.75-2.50% Nb, 0.50-0.70% N, 0-0.30% Si, provided that C+N≧0.8%, the rest consisting mainly of iron and unavoidable impurities. The invention further relates to a method for making said compositions and valves produced from said compositions or using said method and exhibiting excellent mechanical strength and oxidation resistance at temperatures between 800 and 900° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to steel compositions intended in particular forthe manufacture of intake and exhaust valves for vehicles that arepowered by an internal combustion engine.

2. Description of Related Art

During use, this type of part is subjected to major mechanical stressesat temperatures that continue to rise as the power and performance ofthe engines in which they are installed increases. Currently, when theengine intake includes a turbo, this temperature is generally between200 and 400° C., although it can reach 800° C. at the level of theexhaust when the fuel used is gasoline. The exhaust valves can thereforebe subjected to temperatures that range up to 900° C. for each ignitionfollowed by an exhaust. The materials used for these valves must also beable to withstand sudden and large variations in temperature.

This increase in the temperatures of the valves during operation makesthem even more sensitive to oxidation and corrosion caused by certaincomponents of the fuels used, such as lead, sulfur and vanadiumpentoxide, which reduce the useful life of the valves.

The direct oxidation of the metal represents the primary mechanism inthe European countries, where regulations tend to require the use ofunleaded gasoline and a reduction in the amount of sulfur in fuels tovery low levels, on account of atmospheric pollution.

Apart from these various stresses that are encountered during theutilization of the finished parts, the steel or alloy used tomanufacture them must also satisfy certain additional criteria. Thevalves are generally manufactured in two stages. First of all, thesteelmaker will prepare a grade of steel or alloy which it will thensupply to the valve manufacturer in the form of bars that have beenstraightened, but which may also have been rough turned or subjected toany other surface treatment specified by the customer. This manufacturerwill then proceed to shear these bars, in an operation that is alsocalled blank cutting. In an initial manufacturing process, the bar iscut into blanks at a high temperature, which is followed by theextrusion of the blanks into valves at temperatures ranging from 1150 to1200° C., which requires that the grain structure of the bar suppliedremain stable up to the forging temperatures.

In a second manufacturing process, called upsetting, the blanks areobtained by shearing at the ambient temperature, which requires a metalthat is not very brittle, to prevent non-uniform shearing and thecracking of these blanks. It has also been found that during this coldshearing operation, problems can occur that are related to carbidesegregations in the blanks, which can result among other things inexcessive wear of the tools.

The steels of the prior art cause problems during shearing because,among other things, the appearance of cracks in the parts requiresfrequent adjustments to the production lines.

The materials conventionally used for the manufacture of valves of thistype include austenitic stainless steels, which have aniron-nickel-chromium base and range from steels with a high manganesecontent (up to 10% by weight) to steels with a high nickel content (upto 21% by weight). The high-temperature oxidation resistance of thesesteels is not always satisfactory, in particular when, for example, theengine is operating in a marine atmosphere and is exposed to chlorine,or when an increase in the performance of the engine results in hottercombustion gases. These insufficiencies have led steelmakers to increasethe chromium content of their steels even further, which has thedisadvantage that it promotes the formation of ferrite at hightemperature, as well as intermetallic phases that make the steel brittleat engine operating temperatures.

SUMMARY OF THE INVENTION

The essential object of this invention is therefore to eliminate theabove mentioned disadvantages of the steel compositions of the prior artby making available steel compositions that have, among other things,improved resistance to oxidation, improved mechanical characteristicsand improved operational properties that make it possible, among otherthings, to manufacture exhaust valves that have excellent mechanicalstrength and resistance to oxidation in the temperature range from 800to 900° C.

For this purpose, a first object of the invention consists of a steelcomposition which contains, expressed in percentages by weight:

C 0.25-0.35% Cr 24-28% Ni 10-15% Mn 3-6% Nb 1.75-2.50% N 0.50-0.70% Si  0-0.30%

whereby C+N≧0.8%,

and the rest consists primarily of iron and the unavoidable impurities.

In one preferred embodiment of the invention, the steel compositionincludes, expressed in percentages by weight:

C 0.25-0.32% Cr 25-26% Ni 11.50-12.50% Mn 4.80-5.20% Nb 1.90-2.30% N0.61-0.70% Si   0-0.30%

whereby C+N≧0.9%,

and the rest consists primarily of iron and the unavoidable impurities.

The inventors have discovered, to their surprise, that the steelcompositions defined above all have a solidification mode that is veryclose to a eutectic between the y phase of austenite and a phase whichhas been found to he a niobium carbonitride Nb(C,N).

BRIEF DESCRIPTION OF THE DRAWINGS

Three phases diagrams are presented in FIGS. 1 to 3, which correspondrespectively to:

In FIG. 1: Steel compositions not as taught by this invention andcontaining 0.286% C, 4.93% Mn, 11.92% Ni, 25.21% Cr, 0.292% Si, but 1.5%niobium and 0.5% nitrogen,

In FIG. 2: Steel compositions as taught by this invention and identicalto those described above, but containing 1.75% niobium and 0.525%nitrogen,

In FIG. 3: Steel compositions as taught by this invention and identicalto those described above, but containing 2% niobium and 0.55% nitrogen.

FIGS. 4, 6 and 7 represent structures of steels as taught by theinvention at different stages of their preparation and use. FIG. 5 showsthe structure of steel of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The phase diagrams in FIGS. 1-3 have been plotted as a function of thecarbon content of the compositions, because the carbon content must bebetween 0.25 and 0.35% by weight for reasons of hardness, but alsobecause, outside that range, extremely undesirable carbide-basedprecipitates are formed.

An examination of FIG. 1, in which the curve labeled 1 represents theaustenite phase and the curve labeled 7 represents the niobiumcarbonitride phase, reveals that the niobium carbonitride curve exceedsthe austenite curve only for carbon levels that are greater than 0.5% byweight, which implies that the theoretical eutectic γ/Nb(C,N)(represented by the letter E) is located to the right of the diagram.

On the other hand, an examination of FIG. 2, on which the curves arelabeled the same as in FIG. 1, shows that the eutectic is obtained for acarbon content of 0.30% by weight.

When molten steel that has the theoretical eutectic composition iscooled, the niobium carbonitrides that are formed when the temperatureof said eutectic is reached precipitate very early and are thendistributed uniformly in the rest of the molten steel that surroundsthem. The structure that results from the conventional thermomechanicaltransformations, which generally include rolling followed by the coolingof the rolled parts, is uniform and altogether remarkable. Thisstructure is illustrated in FIG. 4. This structure retains its goodhomogeneity throughout the cross section of the bars following heattreatments or reheats at very high temperatures (>1100° C.) asillustrated in FIG. 6.

For purposes of comparison, FIG. 5 also shows the conventional bandedstructure obtained with the austenitic stainless steels of the priorart. These segregated bands are not uniform; the dark bands contain thecarbides while the lighter bands do not. These bands are in factobtained after drawing of the steel, which contains dendrites of theaustenitic phase and an interdentritic and intergranular network ofcarbides that result from an end-of-solidification reaction.

These differences in structure result in major differences in behavior,specifically during the hot transformation of the steel heats that havejust been prepared. If the structure of the steel composition isheterogeneous, as is the case of the compositions of the prior art, thefinal structure of the parts produced will itself be heterogeneous,resulting in variations of the properties of the steel.

Moreover, the heterogeneity of the structure can have otherdisadvantages during the fabrication of the parts. For example, duringthe fabrication of valves for vehicles powered by internal combustionengines, the automaker shears round steel bars that have a diameter of 6to 13 mm on automated production lines. Because the structure of thesteel is not homogeneous, the shearing will not be uniform, whichresults in the appearance of cracks and necessitates frequentadjustments of the production lines.

In one preferred embodiment of the invention, the levels of nitrogen,niobium and carbon in the steel, which are the three elements that formniobium carbonitride Nb(C,N) are selected so that the resulting steelcompositions are hyper-eutectic in the theoretical phase diagrams. Thephase diagrams in FIG. 3 represent one example of a composition of thistype for which the eutectic E corresponds to a carbon level ofapproximately 0.15% by weight. The hyper-eutectic compositions taught bythe invention, for which the carbon concentration is between 0.25 and0.35%, preferably between 0.25 and 0.32% by weight, have the advantagethat the precipitation of the niobium carbonitrides occurs very earlyduring the solidification process, thereby allowing an optimaldistribution of the precipitates within the melt.

It should also be noted that although the compositions can be qualifiedas hyper-eutectic in the theoretical phase diagrams, in industrialpractice the inventors have also noted the primary precipitation of theaustenite phase. This discrepancy between theory and the reality ofexperience can be explained by the phenomena of remelting, germinationand phase growth.

An examination of these diagrams shows one of the advantages of thecompositions taught by the invention, which is that the eutecticγ/Nb(C,N) is retained even with low levels of carbon, because nitrogenis substituted for the carbon in the Nb(C,N) compound. It is thereforepossible to preserve the favorable effect of the eutectic on thesolidification structures, while limiting the level of carbon in thesteel, which has several interesting consequences, as will be explainedbelow.

One of the favorable consequences of the limited levels of carbon isthat there exists a very wide range of temperatures (approximately1,175° C. to 1,300° C.) in which the structure consists exclusively ofaustenite and niobium carbonitrides. In particular, the undesirablecarbide M₂₃C₆ is completely dissolved, which makes possible a goodresponse of the metal during thermomechanical transformation operationssuch as rolling or extrusion/forging.

The presence of niobium carbonitride in this range of temperatures alsohas the advantage of limiting the enlargement of the grains during therecrystallization heat treatments, solution annealing and/or temperingof the finished products. The recrystallized structures are thereforehomogeneous, which is a very valuable characteristic and one which isvery difficult to achieve repeatably when steel compositions of theprior art are used.

The inventors have also sought to limit the level of carbon in thecompositions taught by the invention to reduce the potential level ofgrain boundary precipitation of the undesirable carbide M₂₃C₆ during thefinal stabilization annealing of the pieces or during the utilization ofthe parts made from these steels. This potential level of precipitationnevertheless remains high in the compositions taught by the invention,because nitrogen is substituted for the carbon to form nitrides andcarbonitrides. However, it has been found that, surprisingly, theductility at room temperature, measured by elongation in the A_(5d)tensile test, remains very good. The oxidation resistancecharacteristics are also excellent.

The inventors have also found that the structure obtained uponcompletion of the solidification of the ingots undergoes a majormodification after the conventional thermomechanical transformationoperations (rolling etc.).

It has been found that the network of small rods of eutecticcarbonitrides Nb(C,N) disappears, leaving instead a relatively uniformdistribution of globular carbonitrides Nb(C,N) in the transformedproducts, such as rolled bars, for example, as illustrated in FIG. 6.

When the concentrations of nitrogen, niobium and carbon are such thatthe resulting compositions are hypo-eutectic in the theoretical phasediagrams, the niobium carbonitrides are precipitated only at the end ofthe solidification, resulting in a distribution which is a priori lessadvantageous. The result is what is called a “Chinese script” structure,as illustrated in FIG. 7. Nevertheless, here again, it has surprisinglybeen found that the network of small rods forms globules after forging,which makes possible a subsequent transformation without any particularproblems. On the other hand, the band structure is even more apparent.

The excellent properties observed for the steel compositions taught bythe invention are obtained thanks to the precise balancing of the alloyelements.

The chromium is used essentially to obtain good oxidation resistancethanks to the passivated oxide layer which it forms on the surface ofthe metal. It also has a beneficial influence on the high-temperaturemechanical strength. The level of chromium in the compositions taught bythe invention is from 24 to 28%, preferably 25 to 26% by weight.

Nickel has a desired gamma-forming effect. The quantity of nickel thatcan be used is limited on account of its price to a level that is justsufficient for the solidification of the matrix in the austenite mode.The level of nickel in the compositions taught by the invention is from10 to 15%, preferably 11.5 to 12.5% by weight.

Carbon has the desired hardening effect, but an excessive amount resultsin the precipitation of carbides that increase brittleness and have anadverse effect on oxidation resistance. The level of carbon in thecompositions taught by the invention is from 0.25 to 0.35% by weight,preferably 0.25 to 0.32%.

Nitrogen is a highly effective gamma-forming element which among otherthings makes it possible for the compositions taught by the invention toremain in the austenitic range while retarding the precipitation of theintermetallic phases. However, the level of nitrogen is limited onaccount of the problems encountered in introducing it into steelcompositions on account of its low limit of solubility in molten steel.The level of nitrogen is 0.5 to 0.7%, preferably 0.61 to 0.7% by weight.These levels also correspond to quasi-saturation at equilibrium of theliquid metal at the conventional processing temperatures, which is anadvantage, because this addition is then easy to do with conventionalmeans that are known to technicians skilled in the art. In addition,because the solidification of the steel taught by the invention givesbirth to two phases (austenite and niobium carbonitride) which canaccept a lot of nitrogen, there is no untimely degasification reactionin the ingots which can generate undesirable bubbles or blisters.

Manganese facilitates the introduction of nitrogen into the compositionby increasing the value of its limit of solubility in liquid and solidphases, but the quantity of manganese is limited on account of itsundesirable effects on oxidation resistance. The level of manganese inthe compositions taught by the invention is 3 to 6%, preferably 4.8 to5.2% by weight.

Niobium, in addition to its carbide-forming properties which arefavorable for the high-temperature strength of the steel, makes itpossible to achieve the eutectic described above. The level of niobiumin the compositions taught by the invention is 1.75 to 2.50%, preferably1.90 to 2.30% by weight.

Silicon is limited to a maximum level of 0.30% by weight, although itimproves the oxidation resistance, because it is strongly sigma-formingand also reduces the solubility of nitrogen.

The steel compositions taught by the invention can be used tomanufacture parts according to processes applicable to the conventionalmaterials cited as references, taking these special characteristics intoaccount.

For example, the steels cannot be prepared in a vacuum, because theliquid must be saturated with nitrogen. For this purpose, an electricfurnace or an AOD (Argon Oxygen Decarburization) furnace can be used, orany other suitable means for the preparation of steels that contain highlevels of the alloy element nitrogen, including the secondary refiningprocesses by electroslag remelting. The remelting can be done, forexample, under slag using a consumable electrode if the objective is toachieve a low level of inclusions.

These operations may optionally be followed by a conventional hotthermomechanical transformation process such as forging or rolling,which can in turn be followed by a tempering treatment, which willpreferably be performing by holding the steel at 1,050 to 1,100° C. for1 to 16 hours in air or in another fluid which guarantees a completefine-grain recrystallization and satisfactory ductility characteristics.

The solution annealing and recrystallization annealing treatments aswell as the preheating of products for the manufacture of the valves canbe performed between 1,100 and 1,200° C.; the highest temperaturesresult in limited grain growth.

The purpose of the stabilization annealing is to guarantee a certainstructural and dimensional stability at the temperatures at which thesteel will be used. This treatment can be carried out, for example, inthe form of a hold at 700-1000° C. for 1 to 16 hours in air or inanother fluid. Generally speaking, it is preferable to perform thistreatment at a temperature that is higher than or equal to thetemperature at which the part will be used.

Tests

The symbols used in the following description have the followingmeanings:

R_(m)=maximum strength

R_(p02)=conventional limit of elasticity at 0.2% deformation

A_(sd)=elongation in % on the basis of 5 d (d=diameter of the testpiece).

All the percentages indicated are percentages by weight.

The various tests were performed on one hand on two compositions taughtby the invention designated A and B, and on the other hand on acomposition C which is not a composition that is claimed by this patentand was created specifically for purposes of comparison, as well as onthree reference steel compositions designated D, E and F.

The three reference steels of the prior art are the following:

D: X50CrMnNiNbN 21.9 (DIN 1.4882)

E: X33CrNiMnN 23.8 (DIN 1.4866)

F: X35CrNiMnMoW 25.9

R_(m), R_(p02) and A_(5d) are measured by means of a tensile test.

TABLE 1 A* B* C D E F C 0.30% 0.30% 0.286% 0.52% 0.35% 0.35% Cr 25.46%25.35% 25.21% 20.70% 22.75% 25.50% Ni 12.00% 12.10% 11.92% 3.60% 7.50%9.00% Mn 4.90% 4.84% 4.93% 8.60% 3.25% 5.00% Nb 2.00% 1.98% 1.55% 2.10%— 0.45% N 0.644% 0.55% 0.50% 0.47% 0.275% 0.515% Si 0.22% 0.25% 0.292%0.35% 0.70% 0.18% W — — — 0.99% — 0.725% Mo — — — — — 0.725% V — — — — —0.45% Fe rest rest rest rest rest rest C + N 0.944 0.850 0.786 0.9900.625 0.865 *prepared as taught by the invention

Mechanical Properties at Ambient Temperature and at ElevatedTemperatures

Because the mechanical strength values of the valve steels are verystrongly dependent on the conditions under which they have beenannealed, the values that have been compared below are average valuesobtained for different thermal operating conditions, all including ahigh-temperature solution annealing followed by aging at a lowertemperature.

In addition to the statistical fluctuations of the strength levels fromone lot to another (on the order of several tens of MPa), it was foundthat the elevation of the aging temperature and/or an increase in thehold time at the aging temperature results in a drop in the strengthlevels, in particular in the limit of elasticity, a phenomenon that islinked to the coalescence of the carbides and other precipitates.

That implies that the mechanical strength values measured on a sampletreated by short-term aging at a temperature that is lower than theservice temperature are meaningless, because these values would decreaserapidly during the operational use of the parts manufactured from thesteels in question.

A technician skilled in the art will therefore regard the data in theliterature with some degree of skepticism, in particular when theannealing conditions of the tested pieces are not specified.

That is why the tested compositions were solution annealed at 1,160° C.for 1 hour and then cooled in water, then aged for 4 hours at 850° C.,with the exception of Grade F, which was solution annealed at 1,120° C.for 1 hour, then cooled in water, and then aged at 820° C. for 4 hours.

The aging at 850° C. corresponds to a temperature that is deemed to beequal to or higher than the temperature at which the valves will beoperated in modern engines where the prevailing temperatures are veryhigh.

TABLE 2 Test temperature R_(m) R_(p0.2) A_(5d) Materials (° C.) (MPa)(MPa) (%)  A* ambient 1001 605 26 800° C. 419 263 27 850° C. 348 226 29B* ambient 964 563 26.5 800° C. 394 249 35.5 850° C. 342 226 40 Cambient 957 558 28.5 800° C. 375 234 36 850° C. 298 203 30 D ambient 968555 23.8 800° C. 350 209 41 850° C. 281 187 41 E ambient 916 491 32 800°C. 352 209 51.5 850° C. 286 175 68.5 F ambient 1033 606 24 800° C. 373244 34 850° C. 307 191 49 *prepared as taught by the invention

It has therefore been determined that for aging treatments suitable foruse at very high temperatures, the alloys taught by the inventionexhibit mechanical strength levels that are higher than those of thereference steels, and even more so when the nitrogen level is between0.64% and 0.70% by weight, at least.

Creep-elongation Strength

This strength is determined on the basis of the value of the stress thatresults in 1% elongation by creep in 100 hours.

The three grades A, B and C were previously treated by solutionannealing and aging at 850° C. for 4 hours, while the reference steelgrades were treated in the manner conventional for each steel, which isto their advantage in the comparison.

The results are presented in Table 3.

TABLE 3 Test Stress for 1% temperature elongation Materials (° C.) (MPa)A 815 76 B 815 62 C 815 59 D 815 27 E 815  82* F 815 60

Tests of Resistance to Corrosion and Oxidation

1. Resistance to corrosion by sodium sulfate+NaCl

This test is used to reproduce the conditions experienced by the valveswhen they are in contact with the combustion fumes of Diesel engines ina marine environment, where the corrosion is aggravated by the presenceof chlorides.

The steel test piece is a cylinder 12 mm in diameter and 12 mm long cutin the axis of the products. The test piece is weighed and then placedin a cold alumina crucible which is filled with a mixture of 90% byweight of sodium sulfate and 10% by weight of sodium chloride which haspreviously been melted for 20 minutes in an electric furnace at atemperature of approximately 927° C. The whole test apparatus is left inthe furnace, at that temperature, for one hour.

The test piece is then removed from the crucible and allowed to cool inair. It is then pickled by immersion for approximately 15 minutes in anaqueous solution that has previously been heated to 100° C. andcontaining 12% ferric sulfate and 2.5% of a 40% HF solution. The weightloss is then measured.

The pickling/weighing cycle is repeated several times, and then theweight of the test piece is plotted on a graph as a function of thecumulative duration of the picklings. This graph should show a firststraight line which represents the attack of the oxide formed in contactwith the corrosive mixture, then a second straight line which representsthe attack of the uncorroded steel by the pickling solution. Theintersection of these two straight lines makes it possible to obtain theweight loss of the test piece Am due to corrosion by molten lead oxide.The corrosion rate C is then calculated on the basis of the formulapresented below: $C = \frac{\Delta \quad m}{S \cdot t}$

Δm: Loss of weight of the test piece in g

S: Initial surface area of the test piece in dm²

t: Duration of the corrosion test in hours.

2. Resistance to Oxidation in Air

The steel test piece is a cylinder 6 mm in diameter and 20 mm long cutalong the axis of the products, and with a hole 3 to 4 inches indiameter.

This tests consists of bringing the test piece, which was previouslyplaced in an alumina crucible, to a temperature of 871° C. for 100 hoursin an electric furnace, and then allowing the test piece to cool. Thistest piece is weighed before and after the oxidation, and the weightgain is determined according to the formula presented below:${{Weight}\quad {gain}} = \frac{{Pf} - {Pi}}{S}$

Pf: Weight after oxidation

Pi: Initial weight

S: Initial surface area of the test piece in dm²

Several successive picklings are then performed as for Test 1. The firstpicklings last 10 minutes, and the duration of the pickling is graduallyincreased to 20, 40 and then 60 minutes. The pickling is stopped whenthe uncorroded metal is attached.

The graph of the weight of the test piece as a function of thecumulative duration of the picklings once again gives two straight lineswith different slopes, the intersection of which gives the value of theweight Pr of the uncorroded metal. The weight loss is then calculated asfollows: ${{Weight}\quad {loss}} = \frac{{Pi} - \Pr}{S}$

The results of the corrosion and oxidation tests are presented togetherin the following table:

TABLE 4 Corrosion in the Oxidation in air molten salts Change in weightat Oxide weight - Loss C the end of the test after pickling Materials(g/dm²/h) (g/dm²/100 h) (g/dm²/100 h) A ¹⁾ 0.3 0.120-0.1180 0.80-1.10 B¹⁾ 12.5 −0.020 0.36-0.52 C ¹⁾ 19.2 0.020 0.71-0.78 D ²⁾ — −0.0911.45-2.50 E ²⁾ 0.2 0.047 0.45-0.75 F ²⁾ 63-70 — 0.45-0.60 ¹⁾ Metaltreated by solution annealing and aging for 4 hours at 850° C. ²⁾Conventional treatment, depending on the steel

Although the thermal treatments of these steels are not absolutelyidentical, it has been found that the oxidation rates of the steelstaught by the invention (A and B) are less than those of the referencesteel D, and are on the same order as those of the better steels of theprior art, or even better in the case of Grade B.

An increase in the content of niobium, all other things being equal(C/B), improves the oxidation resistance, because it seems that thenitrogen preferentially forms the nitride NbN rather than the nitrideCr₂N, thereby leaving more chromium free and not fixed.

It is therefore apparent that the steel as taught by the invention canproduce a very good resistance to oxidation in spite of concentrationsof C+N as high as 1%.

The inventors were very surprised to find a very marked improvement incorrosion resistance in the medium Na₂SO₄+NaCl with an increase in thenitrogen content of the steel taught by the invention. When the nitrogencontent is in the highest range, this corrosion resistance in moltensalts is equivalent to that of the best reference steel, in spite of agrain boundary precipitation rate of nitrides and carbides which is muchhigher.

It has therefore been found that the steels taught by the inventionexhibit both excellent mechanical properties at ambient temperature andat very high temperatures, as well as excellent resistance to oxidationand corrosion by molten salts.

It goes without saying that the embodiments of the invention that havebeen described above have been presented purely as examples and are inno way intended to be restrictive, and that numerous modifications caneasily be made by a technician skilled in the art without thereby goingbeyond the context of the invention.

For example, although the principal application of the compositionstaught by the invention described here is the manufacture of valves forvehicles powered by an internal combustion engine, it is clear that theinvention is not limited to such an application, and that it can be usedto manufacture all parts that are required to withstand similar oridentical stresses, as might be the case for hot working tools, forexample, fasteners (screws, nuts) or control mechanisms.

What is claimed is:
 1. A steel composition comprising, expressed inpercentages by weight: C 0.25-0.35% Cr 24-28% Ni 10-15% Mn 3-6% Nb1.75-2.50% N 0.50-0.70% Si   0-0.30%

whereby C+N≧0.8%, and the rest consists primarily of iron and theunavoidable impurities.
 2. The steel composition as claimed in claim 1,wherein the steel composition contains 25 to 26% by weight of chromium.3. The steel composition as claimed in claim 1, wherein the steelcomposition contains 1.90 to 2.30% by weight of niobium.
 4. The steelcomposition as claimed in claim 1, wherein the steel compositioncontains 0.61 to 0.70% by weight of nitrogen.
 5. The steel compositionas claimed in claim 1, whereby C+N≧0.9%.
 6. The steel composition asclaimed in claim 1, wherein the steel composition contains, expressed inpercentages by weight: C 0.25-0.32% Cr 25-26% Ni 11.50-12.50% Mn4.80-5.20% Nb 1.90-2.30% N 0.61-0.70% Si   0-0.30%

whereby C+N≧0.9%, and the rest consists primarily of iron and theunavoidable impurities.
 7. The steel composition as claimed in claim 1,wherein the levels of carbon, nitrogen and niobium are also selected sothat said steel composition is hypereutectic in the theoretical phasediagrams (FIGS. 2 and 3).
 8. A method for the preparation of a steelhaving a composition of: C 0.25-0.35% Cr 24-28% Ni 10-15% Mn 3-6% Nb1.75-2.50% N 0.50-0.70% Si   0-0.30%,

comprising the steps of: preparing a consumable electrode of the saidcomposition; and electroslag remelting the consumable electrode.
 9. Themethod as claimed in claim 8, further including the steps of: solutionannealing the steel at 1100-1200° C.; and heat treating at a temperaturegreater than or equal to the temperature at which said steel will beused.
 10. A part, comprising: a valve having a steel composition of: C0.25-0.35% Cr 24-28% Ni 10-15% Mn 3-6% Nb 1.75-2.50% N 0.50-0.70% Si  0-0.30%,

wherein the steel composition is prepared by: melting a consumableelectrode of the said steel composition; and electroslag remelting theconsumable electrode.
 11. The method as claimed in claim 8, furtherincluding the step of forming the steel by a hot thermomechanicalprocess by one of forging or rolling.
 12. The method as claimed in claim11, further including the step of thermal tempering the steel between1050 and 1100° C., after the step of forming the steel.
 13. The methodas claimed in claim 11, further including the steps of: solutionannealing the steel at 1100-1200° C.; and heat treating at a temperaturegreater than or equal to the temperature at which said piece will beused.
 14. The method as claimed in claim 12, further including the stepsof: solution annealing the steel at 1100-1200° C.; and heat treating ata temperature greater than or equal to the temperature at which thesteel will be used.
 15. The part as claimed in claim 10, wherein thesteel composition is further prepared by: forming the steel by a hotthermomechanical process by one of forging or rolling.
 16. The part asclaimed in claim 15, wherein the steel composition is further preparedby thermal tempering the steel between 1050-1100° C.
 17. The part asclaimed in claim 15, wherein the steel composition is further preparedby solution annealing the steel at 1100-1200° C. and heat treating at atemperature greater than or equal to the temperature at which the steelwill be used.
 18. The part as claimed in claim 16, wherein the steelcomposition is further prepared by solution annealing the steel at1100-1200° C. and heat treating at a temperature greater than or equalto the temperature at which the steel will be used.